nirvana 包

nirvana 包在根目录中，实现了 Nirvana Server 和插件系统。这个包放置在根目录中是因为这个包是 Nirvana 提供的用于生成 API Server 的顶级包，而且其依赖的所有包只来自 Nirvana 自身和标准库（config 包依赖了 nirvana 包和其他第三方的包，实际上是一个借助了 Nirvana 和第三方功能的扩展）。

Server 接口如下：

// Server is a complete API server.
// The server contains a router to handle all requests form clients.
type Server interface {
    // Serve starts to listen and serve requests.
    // The method won't return except an error occurs.
    Serve() error
    // Shutdown gracefully shuts down the server without interrupting any
    // active connections.
    Shutdown(ctx context.Context) error
    // Builder create a service builder for current server. Don't use this method directly except
    // there is a special server to hold http services. After server shutdown, clean resources via
    // returned cleaner.
    // This method always returns same builder until cleaner is called. Then it will
    // returns new one.
    Builder() (builder service.Builder, cleaner func() error, err error)
}

创建 Server 需要使用 Config：

// Config describes configuration of server.
type Config struct {
    ...
}

// Locked checks if the config is locked.
func (c *Config) Locked() bool

// IP returns listenning ip.
func (c *Config) IP() string

// Port returns listenning port.
func (c *Config) Port() uint16

// Logger returns logger.
func (c *Config) Logger() log.Logger

// Configurer is used to configure server config.
type Configurer func(c *Config) error

// Configure configs by configurers. It panics if an error occurs or config is locked.
func (c *Config) Configure(configurers ...Configurer) *Config

// Config gets external config by name. This method is for plugins.
func (c *Config) Config(name string) interface{}

// Set sets external config by name. This method is for plugins.
// Set a nil config will delete it.
func (c *Config) Set(name string, config interface{})

// NewServer creates a nirvana server. After creation, don't modify
// config. Also don't create another server with current config.
func NewServer(c *Config) Server

在 Config 中，存在一些 Server 级别的配置，这些配置是针对当前服务的。而对应的 Configurer 也在当前包中。如果需要对配置进行扩展，增强 Server 功能，则可以增加相应字段，否则应该使用插件机制增加功能。

在 Config 中可以看到 Config 和 Set 方法，这两个方法就是提供给插件允许插件设置自身的配置的。

// ConfigInstaller is used to install config to service builder.
type ConfigInstaller interface {
    // Name is the external config name.
    Name() string
    // Install installs stuffs before server starting.
    Install(builder service.Builder, config *Config) error
    // Uninstall uninstalls stuffs after server terminating.
    Uninstall(builder service.Builder, config *Config) error
}

注册了插件之后，在服务启动的时候，会遍历所有插件配置，然后调用插件的 Install 方法安装插件。